Fifty years of routine in-situ and satellite observations have revealed the progressive deterioration of Antarctica’s most vulnerable regions to climate change: the Antarctic Peninsula and West Antarctic ice sheets. The rapid destabilisation of Larsen A and B ice shelves in the Antarctic Peninsula and the ongoing, potentially irreversible ice losses at Pine Island and Thwaites glaciers, West Antarctica, have been linked to a complex marriage of ocean and atmosphere forcing mechanisms impinging on the continent from the Weddell, Bellingshausen and Amundsen Seas. These phenomena have raised questions about the past and future stability of the ice sheets and water mass properties, and have motivated research focused on elucidating the precise ice-ocean-atmosphere interactions controlling oceanographic and cryospheric change over palaeo- to contemporary timescales. Offshore, similar questions have arisen regarding the role of seabed topography and changing sea ice and oceanographic conditions, and how such phenomena may ultimately impact ice sheet mass-losses.

This session welcomes contributions examining the range of controls driving cryospheric and oceanic change across the Antarctic Peninsula and West Antarctic Ice Sheet regions, as well as those in the wider Weddell Sea sector. Together with model and remotely sensed studies, this session will showcase early results from the International Thwaites Glacier Collaboration and several recent research campaigns conducted in the Weddell Sea.

Co-organized by CL4/OS1
Convener: Christine BatchelorECSECS | Co-conveners: Kiya RivermanECSECS, Frazer ChristieECSECS, Katherine HutchinsonECSECS, David Vaughan (deceased)
| Attendance Wed, 06 May, 14:00–15:45 (CEST)

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Chat time: Wednesday, 6 May 2020, 14:00–15:45

D2459 |
| solicited
Ted Scambos

A warming planet, and particularly the warming Pacific Ocean, has led to major changes in the Larsen-Weddell System. While somewhat less significant than those in the adjacent Amundsen-Bellingshausen Sea and its coastal ice, the changes are nonetheless dramatic indicators of a closely interconnected system, driven by increased westerly winds and their impact on surface melting and ice drift. The system very likely will see further major changes if warming continues through the 22nd Century.

A warming trend in the central Pacific over the past ~80 years has induced air circulation changes over the Southern Ocean and Antarctic Peninsula. A rise in the mean speed of westerly-northwesterly winds across the northern Peninsula led to more frequent foehn events, which in turn increased surface melting on the eastern Peninsula ice shelves, and were responsible for reduced sea ice cover and more frequent shore leads on the eastern edges of the ice shelves. This likely led to greater sub-ice-shelf circulation, possibly including solar-warmed surface water (in summer) and modified Weddell Deep Water (mWDW). Around 1986, structural evidence in the form of more disrupted shear zones and increased rifting suggests that the Larsen A and B ice shelves began to thin and weaken. At this progressed, a combination of increased surface ponding and reduced backstress on the iceshelves led to a series of catastrophic break-ups due to hydrofracture, in 1995 (Larsen A shelf) and 2002 (Larsen B).  More recently, thinning detected by altimetry on the northern Larsen C may have contributed to new fracturing and calving of a large iceberg there in 2016 (iceberg A-68), setting the ice shelf front significantly farther to the west than has previously been observed (since 1898).

Looking forward, if the trend in increased westerly winds and Southern Annular Mode index continues, it is anticipated (modelled) that the large clockwise Weddell Gyre will increase in mean flow speed, and that warm deep water entrained from the Antarctic Circumpolar Current will more frequently mix with the mid- to deep ocean layers in the Weddell Gyre. One outcome of this is likely to be advection of warm deep water into the Ronne Ice Shelf cavity, dramatically increasing the heat available for sub-ice-shelf melting there and potentially changing ice sheet flux from the outlet glaciers significantly.

How to cite: Scambos, T.: Recent Changes in the Larsen-Weddell System , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-603, https://doi.org/10.5194/egusphere-egu2020-603, 2020.

D2460 |
Céline Heuzé and Adriano Lemos

The Weddell Polynya, a large hole in the winter sea ice cover, has intrigued researchers since satellite observations began in the late 70s. There is no consensus regarding the mechanisms leading to its opening, not least because there never was an instrument deployed early enough at the right location and with the right sampling interval. But what if we could predict imminent openings, by detecting early-warning signs from space?

The leading theory among oceanographers is that the polynya opens after the sea ice is melted from below by upwelled warm waters. We argue that such upwelling, or at least the increased heat flux through a thinning ice, should be visible on spaceborne thermal infrared imagery. Using microwave-based sea ice products to determine past polynya openings, we first found that there were in fact 83 Weddell / Maud Rise Polynya occurrences since winter 2000, 19 of which reaching an area larger than 1000 km2. We then created a timeseries of (cloud-filtered) daily mean brightness temperature at 3.7, 10.5 and 12 μm from Advanced Very High Resolution Radiometer datasets and found a significant warm temperature anomaly at least 10 days before the polynya opened, peaking at 4K for all bands 5-6 days before the opening. The anomaly is on average 2K stronger for the large polynyas (> 1000 km2). Moreover, the band ratios brutally change magnitude, which suggests lead formation rather than progressive melting – a hypothesis that would agree with meteorologists' theory that the polynya opens because of winds, and that we are now checking with spaceborne radar.

Six days is not much, but it would be enough to re-route expeditions or autonomous sensors so that the opening can be monitored in details. And this is only the first step of our ongoing project... stay tuned to see if we can predict weeks or months ahead!

How to cite: Heuzé, C. and Lemos, A.: Imminent re-opening of the Weddell Polynya detectable days ahead by spaceborne infrared , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3636, https://doi.org/10.5194/egusphere-egu2020-3636, 2020.

D2461 |
Martin Mohrmann, Céline Heuzé, and Sebastiaan Swart

The presence of polynyas has a large effect on air-sea fluxes and deep water production, therefore impacting climate-relevant properties such as heat and carbon exchange between the atmosphere and ocean interior. One of the key areas of deep water formation is in the Weddell Sea, where much attention has recently been placed in the reoccurance of the open ocean Maud Rise polynya. In this study, two methods are presented to track the number, area and spatial distribution of polynyas with a focus on the Weddell Sea. The analysis is applied to a set of 10 Coupled Model Intercomparison Project phase 6 (CMIP6) models and to satellite sea ice concentration data. The first approach is a sea ice threshold method applied to the CMIP6 sea ice data at the original model grid. Open water areas surrounded by sea ice are classified as polynyas. Without requiring any remapping or interpolation, this method preserves the area information of all grid cells and is well suited to compute the combined area of the polynyas in the Weddell Sea. The second approach makes use of an image analysis technique to outline areas with low sea ice concentration. This method is preferable for counting the absolute number of polynyas and obtaining statistical information about their position. Satellite sea ice concentration is used as a reference to compare the performance of the models representing polynya area and to indicate model biases in the location of polynyas. All analyzed CMIP6 models show coastal polynyas, while only about half of the models regularly form open water polynyas. The resolution (about one degree for most models) sets a limit for the number of the polynyas in the numerical models.

How to cite: Mohrmann, M., Heuzé, C., and Swart, S.: Polynya area and frequency in the Weddell Sea in CMIP6 climate models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19812, https://doi.org/10.5194/egusphere-egu2020-19812, 2020.

D2462 |
Markus Janout, Hartmut Hellmer, Tore Hattermann, Svein Osterhus, Lucrecia Stulic, Oliver Huhn, Jürgen Sültenfuss, and Torsten Kanzow

The Filchner and Ronne ice sheets (FRIS) compose the second largest contiguous ice sheet on the Antarctic continent. Unlike at several other Antarctic glaciers, basal melt rates at FRIS are comparatively low, as cold and dense waters presently dominate the wide southern Weddell Sea (WS) continental shelf and effectively block out any significant inflow of warmer ocean waters. We revisited the southern WS shelf in austral summer 2018 during Polarstern expedition PS111 with detailed hydrographic and tracer measurements along both the Ronne and Filchner ice fronts. The hydrography along FRIS was characterized by near-freezing high salinity shelf water (HSSW) in front of Ronne, and a striking dominance of ice shelf water (ISW) in Filchner Trough. The cold (-2.2°C) and fresher (34.6) ISW was formed by the interaction of Ronne-sourced HSSW with the ice shelf base. The strong dominance of ISW in Filchner Trough indicates a recently enhanced circulation below FRIS, likely fueled by enhanced sea ice production in the southwestern WS. We view these recent observations in a multidecadal (1973-present) context, contrast the two dominant circulation modes below FRIS, and discuss the importance of sea ice formation and large-scale sea level pressure patterns for the stability of the ocean circulation and basal melt rates underneath FRIS.

How to cite: Janout, M., Hellmer, H., Hattermann, T., Osterhus, S., Stulic, L., Huhn, O., Sültenfuss, J., and Kanzow, T.: A recent update on the circulation and water masses around the Filchner and Ronne ice shelves in the southern Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10194, https://doi.org/10.5194/egusphere-egu2020-10194, 2020.

D2463 |
Anna Wåhlin, Bastien Queste, Alastair Graham, Kelly Hogan, Lars Boehme, Karen Heywood, Robert Larter, Erin Pettit, and Julia Wellner

The fate of the West Antarctic Ice Sheet is the largest remaining uncertainty in predicting sea-level rise through the next century, and its most vulnerable and rapidly changing outlet is Thwaites Glacier . Because the seabed slope under the glacier is retrograde (downhill inland), ice discharge from Thwaites Glacier is potentially unstable to melting of the underside of its floating ice shelf and grounding line retreat, both of which are enhanced by warm ocean water circulating underneath the ice shelf. Recent observations show surprising spatial variations in melt rates, indicating significant knowledge gaps in our understanding of the processes at the base of the ice shelf. Here we present the first direct observations of ocean temperature, salinity, and oxygen underneath Thwaites ice shelf collected by an autonomous underwater vehicle, a Kongsberg Hugin AUV. These observations show that while the western part of Thwaites has outflow of meltwater-enriched circumpolar deep water found in the main trough leading to Thwaites, the deep water (> 1000 m) underneath the central part of the ice shelf is in connection with Pine Island Bay - a previously unknown westward branch of warm deep water flow. Mid-depth water (700 - 1000 m) enters the cavity from both sides of a buttressing point and large spatial gradients of salinity and temperature indicate that this is a region of active mixing processes. The observations challenge conceptual models of ice-ocean interactions at glacier grounding zones and identify a main buttressing point as a vulnerable region of change currently under attack by warm water inflow from all sides: a scenario that may lead to ungrounding and retreat more quickly than previously expected.

How to cite: Wåhlin, A., Queste, B., Graham, A., Hogan, K., Boehme, L., Heywood, K., Larter, R., Pettit, E., and Wellner, J.: Warm water flow and mixing beneath Thwaites Glacier ice shelf, West Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19934, https://doi.org/10.5194/egusphere-egu2020-19934, 2020.

D2464 |
Yoshihiro Nakayama, Georgy Manucharyan, Hong Zhang, Pierre Dutrieux, Hector S. Torres, Patrice Klein, Helene Seroussi, Michael Schodlok, Eric Rignot, and Dimitris Menemenlis

In the Amundsen Sea, modified Circumpolar Deep Water (mCDW) intrudes into ice shelf cavities, causing high ice shelf melting near the ice sheet grounding lines, accelerating ice flow, and controlling the pace of future Antarctic contributions to global sea level. The pathways of mCDW towards grounding lines are crucial as they directly control the heat reaching the ice. A realistic representation of mCDW circulation, however, remains challenging due to the sparsity of in-situ observations and the difficulty of ocean models to reproduce the available observations. In this study, we use an unprecedentedly high-resolution (200 m horizontal and 10 m vertical grid spacing) ocean model that resolves shelf-sea and sub-ice-shelf environments inqualitative agreement with existing observations during austral summer conditions. We demonstrate that the waters reaching the Pine Island and Thwaites grounding lines follow specific, topographically-constrained routes, all passing through a relatively small area located around 104ºW and 74.3ºS. The temporal and spatial variabilities of ice shelf melt rates are dominantly controlled by the sub-ice shelf ocean current. Our findings highlight the importance of accurate and high-resolution ocean bathymetry and subglacial topography for determining mCDW pathways and ice shelf melt rates. 

We also briefly introduce our various existing model outputs focusing on the Amundsen Sea and demonstrate how to access these model outputs, plot some basic variables, and create animations. We hope that these model output can be utilized for many different aspects of oceanographic researches including observational planning, data analysis for physical, biological and chemical oceanography, and boundary conditions for ocean and ice sheet models. 


How to cite: Nakayama, Y., Manucharyan, G., Zhang, H., Dutrieux, P., S. Torres, H., Klein, P., Seroussi, H., Schodlok, M., Rignot, E., and Menemenlis, D.: Pathways of ocean heat towards Pine Island and Thwaites grounding lines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-245, https://doi.org/10.5194/egusphere-egu2020-245, 2020.

D2465 |
| Highlight
David Lilien, Ian Joughin, Benjamin Smith, and Noel Gourmelen

Smith, Pope, and Kohler glaciers and the corresponding Crosson and Dotson ice shelves have undergone speedup, thinning, and rapid grounding-line retreat in recent years, leaving them in a state likely conducive to future retreat. We conducted a suite of numerical model simulations of these glaciers and compared the results to observations to determine the processes controlling their recent evolution. Simulations were forced using estimates of the distribution and intensity of melt from 1996-2014. The model simulations indicate that the state of these glaciers in the 1990s was not inherently unstable, i.e., that small perturbations to the grounding line would not necessarily have caused the large retreat that has been observed. Instead, sustained melt, at rates higher than the 1990s and concentrated at the grounding line, was needed to cause the observed retreat. Weakening of the margins of Crosson Ice Shelf may have hastened the onset of grounding-line retreat but is unlikely to have initiated these rapid changes without an accompanying increase in melt. In the simulations that most closely match the observed thinning, speedup, and retreat, modeled grounding-line retreat and ice loss continue unabated throughout the 21st century, and subsequent retreat along Smith Glacier's trough appears likely. Given the modeled retreat, thinning associated with the retreat of Smith Glacier may reach the ice divide and undermine a portion of the Thwaites catchment as quickly as changes initiated at the Thwaites terminus. Thus, while the Smith, Pope, Kohler catchment is small compared to Thwaites, these smaller glaciers may be important when considering the centennial-scale evolution of the Amundsen Sea region.

How to cite: Lilien, D., Joughin, I., Smith, B., and Gourmelen, N.: Melt at grounding line controls observed and future retreat of Smith, Pope, and Kohler Glaciers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8411, https://doi.org/10.5194/egusphere-egu2020-8411, 2020.

D2466 |
Ilka Peeken, Stefanie Arndt, Markus Janout, Thomas Krumpen, and Christian Haas

The western Weddell Sea along the northward branch of the Weddell Gyre is a region of major outflow of various water masses, thick sea ice, and biogeochemical matter, linking the Antarctic continent to the world oceans. It features a deep shelf and the second largest ice shelf (Larsen C) in the WS, and its perennial sea ice cover is among the thickest on earth. This region is undergoing dramatic changes due to the breakup of ice shelves along the Antarctic Peninsula, which results in oceanographic conditions unprecedented in the past 10,000 years. Since this region is difficult to access, comprehensive physical and biogeochemical information is still lacking. During the interdisciplinary Weddell Sea Ice (WedIce) expedition to the northwestern Weddell Sea on board the German icebreaker RV Polarstern in spring 2019, oceanographic and biogeochemical studies were conducted together with in-situ snow and ice sampling. Most stations visited contained second- and third-year ice. Additional airborne ice-thickness surveys revealed a mean ice thicknesses between 2.6 and 5.4 m, increasing from the Antarctic Sound towards the Larsen B region. Usually rotten ice was present below a solid, ~30 cm thick surface-ice layer, however, pronounced gap layers, typical for late summer ice in the marginal ice zone, were rare. The associated high algal biomass was only found north of the Antarctic Sound. Nevertheless, diatom-dominated standing stocks of integrated sea ice algae biomass were among the highest, previously described in Antarctic waters. In contrast, despite overall high macro-nutrient concentrations in the water, the biomass of the flagellate dominated phytoplankton was negligible for primary production in the entire region. Overall, it seems that despite changing light conditions for the phytoplankton due to the loss of ice shelves, the sea ice-derived carbon represents an important control variable for higher trophic levels in the western Weddell Sea.

How to cite: Peeken, I., Arndt, S., Janout, M., Krumpen, T., and Haas, C.: The importance of sea ice biota for the ecosystem in the northwestern Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20152, https://doi.org/10.5194/egusphere-egu2020-20152, 2020.

D2467 |
Stefanie Arndt, Christian Haas, and Ilka Peeken

Summer sea ice extent in the Weddell Sea has increased overall during the last four decades, with large interannual variations. However, the underlying causes and the related ice and snow properties are still poorly known. Here we present results of the interdisciplinary Weddell Sea Ice (WedIce) project carried out in the northwestern Weddell Sea on board the German icebreaker R/V Polarstern in February and March 2019, i.e. at the end of the summer ablation period. This is the region of the thickest, oldest ice in the Weddell Sea, at the outflow of the Weddell Gyre. Measurements included airborne ice thickness surveys and in-situ snow and ice sampling of mostly second- and third year ice. Preliminary results show mean ice thicknesses between 2.6 and 5.4 m, increasing from the Antarctic Sound towards the Larsen B region. The ice had mostly positive ice freeboard. Mean snow thicknesses ranged between 0.05 and 0.46 m. Snow was well below the melting temperature on most days and was highly metamorphic and icy, with melt-freeze forms as dominant snow type. In addition, as a result of the summer’s thaw, an average of 0.14 m of superimposed ice was found in all ice cores drilled during the cruise. Although there was rotten ice below a solid, ca. 30 cm thick surface ice layer, pronounced gap layers typical for late summer ice in the marginal ice zone were rare, and algal biomass was patchily distributed within individual sea ice cores. Overall, there was a strong gradient of increasing ice algal biomass from the Larsen B to the Antarctic Sound region. The presented results show that sea ice conditions in the northwestern Weddell Sea are still severe and have not changed significantly since the last observations carried out in 2004 and 2006. The presence of relatively thin, icy snow has strong implications for the ice and snow mass balance, for freshwater oceanography, and for the application of remote sensing methods. Overall sea ice properties strongly affect the biological productivity of this region and limit carbon fluxes to the seafloor in the northwestern Weddell Sea.

How to cite: Arndt, S., Haas, C., and Peeken, I.: New observations of late summer bio-physical ice and snow conditions in the northwestern Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-253, https://doi.org/10.5194/egusphere-egu2020-253, 2020.

D2468 |
| solicited
Kelly Hogan, Robert Larter, Alastair Graham, Robert Arthern, James Kirkham, Rebecca Totten, Tom Jordan, Rachel Clark, Victoria Fitzgerald, John Anderson, Claus-Dieter Hillenbrand, Frank Nitsche, Lauren Simkins, James Smith, Karsten Gohl, Jan Erik Arndt, Jongkuk Hong, and Julia Wellner

The coastal bathymetry of Thwaites Glacier (TG) is poorly known yet nearshore sea-floor highs have the potential to act as pinning points for floating ice shelves, or to block warm water incursions to the grounding line. In contrast, deeper areas control warm water routing. Here, we present more than 2000 km2 of new multibeam echo-sounder data (MBES) acquired offshore TG during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project on the RV/IB Nathaniel B. Palmer (NBP19-02) in February-March 2019. Beyond TG, the bathymetry is dominated by a >1200 m deep, structurally-controlled trough and discontinuous ridge, on which the Eastern Ice Shelf is pinned. The geometry and composition of the ridge varies spatially with some sea-floor highs having distinctive flat-topped morphologies produced as their tops were planed-off by erosion at the base of the seaward-moving Thwaites Ice Shelf. In addition, submarine landform evidence indicates at least some unconsolidated sediment cover on the highs, as well as in the troughs that separate them. Knowing that this offshore area of ridges and troughs is a former bed for TG, we also used a novel spectral approach and existing ice-flow theory to investigate bed roughness and basal drag over the newly-revealed offshore topography. We show that the sea-floor bathymetry is a good analogue for extant bed areas of TG and that ice-sheet retreat over the sea-floor troughs and ridges would have been affected by high basal drag similar to that acting in the grounding zone today.

Comparisons of the new MBES data with existing regional compilations show that high-frequency (finer than 5 km) bathymetric variability beyond Antarctic ice shelves can only be resolved by observations such as MBES and that without these data calculations of the oceanic heat flux may be significantly underestimated. This work supports the findings of recent numerical ice-sheet and ocean modelling studies that recognise the need for accurate and high-resolution bathymetry to determine warm water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.

How to cite: Hogan, K., Larter, R., Graham, A., Arthern, R., Kirkham, J., Totten, R., Jordan, T., Clark, R., Fitzgerald, V., Anderson, J., Hillenbrand, C.-D., Nitsche, F., Simkins, L., Smith, J., Gohl, K., Arndt, J. E., Hong, J., and Wellner, J.: Lessons learnt from the former bed of Thwaites Glacier: a new multibeam-bathymetric dataset, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17323, https://doi.org/10.5194/egusphere-egu2020-17323, 2020.

D2469 |
Jonathan Bamber and Geoffrey Dawson

Pine Island Glacier (PIG) has contributed more to sea level rise over the last four decades than any other glacier in Antarctica. Model projections indicate that this will continue in the future but at conflicting rates, depending partly on the model initialisation. Some models suggest that mass loss could dramatically increase over the next few decades, resulting in a rapidly growing contribution to sea level, and fast retreat of the grounding line. Other models indicate more moderate losses. Resolving this contrasting behaviour is important for sea level rise projections as PIG and the Amndsen Sea Sector have been used as calibration for plausibility, probabilistic and deterministic approaches. Here, we use high resolution satellite observations of elevation change since 2010 from CryoSat-2 swath data to show that thinning rates are now highest along the slow-flow margins of the glacier and that the present-day amplitude and pattern of elevation change is inconsistent with fast grounding line migration and the associated rapid increase in mass loss over the next few decades. Instead, our results support model simulations that imply only modest changes in grounding line location over that timescale. We demonstrate how the pattern of thinning is evolving in complex ways both in space and time and how rates in the fast-flowing central trunk have decreased by about a factor five since 2007. We also consider how the complex pattern of mass loss affects the interpretation of vertical land motion from GPS data and inferences made from these data for mantle viscosity and solid Earth response times.


How to cite: Bamber, J. and Dawson, G.: Complex, evolving patterns of mass loss from Antarctica’s largest glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1567, https://doi.org/10.5194/egusphere-egu2020-1567, 2020.

D2470 |
Elisabeth Clyne, Sridhar Anandakrishnan, Atsuhiro Muto, Richard Alley, Donald Voight, and Kiya Riverman

Thwaites Glacier (TG), West Antarctica, is losing mass in response to oceanic forcing. Future evolution could lead to deglaciation of the marine basins of the West Antarctic ice sheet, depending on ongoing and future climate forcings, but also on basal topography/bathymetry, basal properties, and physical processes operating within the grounding zone. Hence, it is important to know the actual distribution of bed types of TG’s interior and grounding zone, and to incorporate them accurately in models to improve estimates of retreat rates and stability. Here we determine bed reflectivity and acoustic impedance via amplitude analysis of reflection seismic data. We report on the results from two lines – a longitudinal (L-Line) and a transverse (N-Line) – on a central flowline of TG 100 km inland from the grounding zone. There is considerable variability in bed forms and properties, both within this dataset and in-comparison with nearby work. Notably, we find the same hard (bedrock) stoss and soft (till) lee pattern observed elsewhere on TG in prior work. Physical understanding indicates the basal flow law describing motion over different regions of TG’s bed likely varies from nearly viscous over the hard bedrock regions to nearly plastic over soft till regions, providing a template for modeling.

How to cite: Clyne, E., Anandakrishnan, S., Muto, A., Alley, R., Voight, D., and Riverman, K.: Reflection Seismic Interpretation of Topography and Acoustic Impedance beneath Thwaites Glacier, West Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1304, https://doi.org/10.5194/egusphere-egu2020-1304, 2020.

D2471 |
| Highlight
Julian Dowdeswell, Christine Batchelor, Sasha Montelli, Dag Ottesen, Evelyn Dowdeswell, and Jeffrey Evans

Multibeam echo-sounders were deployed from Autonomous Underwater Vehicles (AUVs) flying close to the seafloor of the Weddell Sea shelf in order to investiagte the glacial landforms there with a view to understanding processes and patterns associated with deglaciation from the Last Glacial Maximum on the eastern side of the Antarctic Peninsula. A horizontal resolution of 0.5 m (using conventional mulitbeam systems), and in some cases 0.05 m (using interferometric multibeam equipment), allowed delicate seafloor landforms to be mapped in several areas of the shelf beyond the Larsen C and former Larsen A and B ice shelves. A number of glacial landform assemblages were observed, including suites of delicate ridges associated with grounding-zone wedges and the grounding of icebergs on the shelf. These landforms are probably related to the action of tides moving the ice up and down through a series of tidal cycles. At the highest spatial resolution, individual dropstones derived from rain-out during the melting of floating ice were imaged clearly. Imaging the seafloor at such high resolution allows both very detailed descriptions of submarine landform morphology and also the complexity of such landforms and landform assemblages to be better understood, aiding the interpretation of the glacial and related processes that led to their formation.

How to cite: Dowdeswell, J., Batchelor, C., Montelli, S., Ottesen, D., Dowdeswell, E., and Evans, J.: Submarine landforms on the Weddell Sea shelf imaged at high resolution using AUVs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1603, https://doi.org/10.5194/egusphere-egu2020-1603, 2020.

D2472 |
| Highlight
Britney Schmidt, Keith Nicholls, Peter Davis, James Smith, Kiya Riverman, David Holland, Daniel Dichek, Andrew Mullen, Justin Lawrence, Peter Washam, Aurora Basinski-ferris, Paul Anker, Matthew Meister, Anthony Spears, Ben Hurwitz, Enrica Quartini, Elisabeth Clyne, Catrin Thomas, James Wake, and David Vaughn and the ITGC Team (MELT & Thwaites Glacier Grounding Zone Downstream teams)

Icefin performed the first long range robotic exploration of the grounding zone of Thwaites Glacier from January 9-12 2020. Icefin was part of the MELT project of the International Thwaites Glacier Collaboration deployed to the grounding zone of Thwaites Glacier, West Antarctica over the period December 2019-February 2020.  MELT is an interdisciplinary project to explore rapid change across the grounding zone, and in particular basal melting.  The subglacial cavity ~2km north of the grounding zone was accessed via hot water drilling on January 7-8, 2020.  Icefin, a hybrid autonomous and remotely operated underwater vehicle designed for sub-ice and borehole operations, conducted over 15km of round-trip data collection under the ice along a section of the glacier from the grounding zone extending to a point 4 km oceanward.   The vehicle collected data with ten different science sensors including cameras, sonars, conductivity/temperature and dissolved oxygen.  Overall, the water column ranged from ~100m downstream that narrowed quickly to an average of 50m that spanned over 2km, to a long segment of ~30m thickness before quickly narrowing over 500m towards the grounding zone. The seafloor structures run roughly parallel to ice flow direction, consisting of furrows, ridges, and grooves in some cases mirrored by the ice structure. The Icefin dives revealed a diverse set of basal ice conditions, with complex geometry, including a range of terraced features, smooth ablated surfaces, crevassing, sediment rich layers of varying kinds, as well as interspersed clear, potentially accreted freshwater ice.  The ocean directly beneath the ice varies spatially, from moderately well-mixed near the grounding zone to highly stratified within and below concavities in the ice downstream.  Sediments along the sea floor range from fine grained downstream to course angular gravel near the grounding zone distributed between larger boulders.  We observed rocky material in the ice that ranged from fine grained layers compressed within the ice to small angular particles volumetrically distributed within ice, to gravel and cobbles, as well as trapped boulders up to meter scale. In addition to the oceanographic, glaciological and sea floor conditions, we also catalogued communities of organisms along the seafloor and ice-ocean interface. We will report the highlights and initial conclusions from Icefin’s in situ data collection, and offer perspectives on change at the grounding zone.

How to cite: Schmidt, B., Nicholls, K., Davis, P., Smith, J., Riverman, K., Holland, D., Dichek, D., Mullen, A., Lawrence, J., Washam, P., Basinski-ferris, A., Anker, P., Meister, M., Spears, A., Hurwitz, B., Quartini, E., Clyne, E., Thomas, C., Wake, J., and Vaughn, D. and the ITGC Team (MELT & Thwaites Glacier Grounding Zone Downstream teams): The Grounding Zone of Thwaites Glacier Explored by Icefin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20512, https://doi.org/10.5194/egusphere-egu2020-20512, 2020.

D2473 |
Toby Benham, Frazer Christie, and Julian Dowdeswell

The distribution and concentration of sea ice presents a significant challenge to shipping and scientific expeditions in high-latitude regions. In addition to achieving safe navigation, information about likely sea ice conditions is needed for expedition planning, and the deployment and retrieval of scientific instruments and their data. In areas where time series of passive microwave data exist, broad-scale analysis of sea ice concentration can be readily achieved. However, the spatial resolution of these data does not permit detailed investigations of sea ice conditions, including near-shore lead development.

Here we present a new methodology for processing moderate resolution multispectral and thermal satellite imagery to summarise inter-annual differences in the probability of sea ice observation. By using multiple daily imagery sources (Terra and Aqua MODIS; Suomi-NPP VIIRS), and averaging resultant concentration maps over longer time periods, we reduce the impediment of cloud cover to characterising sea ice using this type of imagery. Our processing provides a higher-resolution depiction of sea ice conditions and their variability than that afforded by passive microwave data. By estimating a sub-pixel concentration for all pixels identified as ‘Ice’, we capture further nuances of narrower water/thin ice inclusions within the ice cover.

The utility of this new methodology to support operational ship survey in polar regions is demonstrated using examples from the Weddell Sea, Antarctica. Our description of sea ice cover agrees well with that derived from very high-resolution imagery from the Operation Ice Bridge DMS camera system, and with experience of the actual sea ice conditions encountered during the Weddell Sea Expedition in early 2019.

How to cite: Benham, T., Christie, F., and Dowdeswell, J.: Sea ice in the Weddell Sea: use of moderate resolution imagery to summarize inter-annual variation in conditions and support operational ship survey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21821, https://doi.org/10.5194/egusphere-egu2020-21821, 2020.

D2474 |
John Shears, Julian Dowdeswell, Freddie Ligthelm, and Paul Wachter

The Weddell Sea Expedition 2019 (WSE) was conceived with dual aims: (i) to undertake a comprehensive international inter-disciplinary programme of science centred in the waters around Larsen C Ice Shelf, western Weddell Sea; and (ii) to search for, survey and image the wreck of Sir Ernest Shackleton’s Endurance, which sank in the Weddell Sea in 1915. 

The 6-week long expedition, funded by the Flotilla Foundation, required the use of a substantial ice-strengthened vessel given the very difficult sea-ice conditions encountered in the Weddell Sea, and especially in its central and western parts. The South African ship SA Agulhas II was chartered for its Polar Class 5 icebreaking capability and design as a scientific research vessel. The expedition was equipped with state-of-the-art Autonomous Underwater Vehicles (AUVs) and a Remotely Operated Vehicle (ROV) which were capable of deployment to waters more than 3,000 m deep, thus making the Larsen C continental shelf and slope, and the Endurance wreck site, accessible. During the expedition, a suite of passive and active remote-sensing data, including TerraSAR-X radar images delivered in near real-time, was provided to the ice-pilot onboard the SA Agulhas II. These data were instrumental for safe vessel navigation in sea ice and the detection and tracking of icebergs and ice floes of scientific interest.

The scientific programme undertaken by the WSE was very successful and produced many new geological, geophysical, marine biological and oceanographic observations from a part of the Weddell Sea that has been little studied previously, particularly the area east of Larsen C Ice Shelf. The expedition also reached the sinking location of Shackleton’s Endurance, where the presence of open-water sea ice leads allowed the deployment of an AUV to the ocean floor to try and locate and survey the wreck. Unfortunately, SA Agulhas II later lost communication with the AUV, and deteriorating weather and sea ice conditions meant that the search had to be called off.

How to cite: Shears, J., Dowdeswell, J., Ligthelm, F., and Wachter, P.: The Weddell Sea Expedition 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21896, https://doi.org/10.5194/egusphere-egu2020-21896, 2020.

D2475 |
Frazer Christie, Toby Benham, and Julian Dowdeswell

The Antarctic Peninsula is one of the most rapidly warming regions on Earth. There, the recent destabilization of the Larsen A and B ice shelves has been directly attributed to this warming, in concert with anomalous changes in ocean circulation. Having rapidly accelerated and retreated following the demise of Larsen A and B, the inland glaciers once feeding these ice shelves now form a significant proportion of Antarctica’s total contribution to global sea-level rise, and have become an exemplar for the fate of the wider Antarctic Ice Sheet under a changing climate. Together with other indicators of glaciological instability observable from satellites, abrupt pre-collapse changes in ice shelf terminus position are believed to have presaged the imminent disintegration of Larsen A and B, which necessitates the need for routine, close observation of this sector in order to accurately forecast the future stability of the Antarctic Peninsula Ice Sheet. To date, however, detailed records of ice terminus position along this region of Antarctica only span the observational period c.1950 to 2008, despite several significant changes to the coastline over the last decade, including the calving of giant iceberg A-68a from Larsen C Ice Shelf in 2017.

Here, we present high-resolution, annual records of ice terminus change along the entire western Weddell Sea Sector, extending southwards from the former Larsen A Ice Shelf on the eastern Antarctic Peninsula to the periphery of Filchner Ice Shelf. Terminus positions were recovered primarily from Sentinel-1a/b, TerraSAR-X and ALOS-PALSAR SAR imagery acquired over the period 2009-2019, and were supplemented with Sentinel-2a/b, Landsat 7 ETM+ and Landsat 8 OLI optical imagery across regions of complex terrain.

Confounding Antarctic Ice Sheet-wide trends of increased glacial recession and mass loss over the long-term satellite era, we detect glaciological advance along 83% of the ice shelves fringing the eastern Antarctic Peninsula between 2009 and 2019. With the exception of SCAR Inlet, where the advance of its terminus position is attributable to long-lasting ice dynamical processes following the disintegration of Larsen B, this phenomenon lies in close agreement with recent observations of unchanged or arrested rates of ice flow and thinning along the coastline. Global climate reanalysis and satellite passive-microwave records reveal that this spatially homogenous advance can be attributed to an enhanced buttressing effect imparted on the eastern Antarctic Peninsula’s ice shelves, governed primarily by regional-scale increases in the delivery and concentration of sea ice proximal to the coastline.

How to cite: Christie, F., Benham, T., and Dowdeswell, J.: Recent glacial advance in the western Weddell Sea Sector driven by anomalous sea ice circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7801, https://doi.org/10.5194/egusphere-egu2020-7801, 2020.

D2476 |
Wolfgang Rack, Frazer Christie, Evelyn Dowdeswell, Julian Dowdeswell, Paul Wachter, Toby Benham, Christian Haas, and Paul Bealing

The 2019 Weddell Sea expedition provided a unique opportunity for geophysical and glaciological sea ice measurements in one of the least accessible regions of the Southern Ocean. Although the extent and area of sea ice is well known based on satellite measurements, the limited information on thickness does still hinder the calculation of trends trends in volume and mass. Sea ice thickness is therefore one of the missing key variables in the global cryosphere mass balance, and difficult logistics are a challenge for near synchronous satellite validation measurements. Another key variable in this context is snow on sea ice, as knowledge of snow is required to convert satellite-derived freeboard to thickness.

We measured the sea-ice morphology by a combination of on ice and remote sensing methods: near-synchronous temporal and spatial measurements from a drone equipped with a radar sensor and camera, manually-derived on-ice surveys and samples such as snow pits, snow-depth transects and drill holes, and a AUV with upward-looking multibeam sonars. We also deployed ice-drifter buoys on several ice floes which we used to provide floe drift over an extended period of time.

In this contribution we present the results of our observations in conjunction with a close sequence of high resolution satellite radar images (TerraSAR-X, Sentinel-1) and altimeter data (ICESat-2 and CryoSat-2) to characterise the sea ice conditions in the western Weddell Sea. We found a mixture of fragments of deformed first-year and multi-year sea-ice which was consolidated in larger ice floes. A thick snow cover frequently depressed the ice cover of the thinner first year ice below sea level. Satellite data allow to extend our findings in time to a larger area and to improve our information on sea ice over a larger region.

How to cite: Rack, W., Christie, F., Dowdeswell, E., Dowdeswell, J., Wachter, P., Benham, T., Haas, C., and Bealing, P.: Sea ice characteristics during the Weddell Sea expedition explored by geophysical and remote sensing methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20610, https://doi.org/10.5194/egusphere-egu2020-20610, 2020.

D2477 |
Ted Scambos, Jennifer Bohlander, and Karen Alley

Crane and Hektoria glaciers, the major tributaries of the former Larsen B Ice Shelf, underwent major structural and ice flow changes in the aftermath of the ice shelf’s disintegration in March, 2002. In addition to the widely reported initial acceleration (leading to speeds 3 to 6 times the pre-disintegration rate), the continued retreat led to the formation of significant ice cliffs. For Hektoria, this occurred as a seamless transition from ice shelf disintegration. Crane Glacier had a two-stage acceleration, first increasing in speed by 3x in the first few months after disintegration, then slowing through September 2004, and then a rapid additional acceleration in 2005-2006. Both glaciers developed significant ice cliffs during retreat, with peak ice-front heights of 105 m for Crane and 85 m for Hektoria.

How to cite: Scambos, T., Bohlander, J., and Alley, K.: Post-disintegration evolution of the largest Larsen B tributary glaciers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1330, https://doi.org/10.5194/egusphere-egu2020-1330, 2020.

D2478 |
Joshua Mirkin, Adam West, Katherine Hutchinson, Raquel Flynn, and Sarah Fawcett

The Larsen C ice shelf (LCIS) in the western Weddell Sea has recently undergone large-scale ice shelf collapse with the detachment of iceberg A68 in 2017. Cold cavity ice shelves, such as LCIS, are critical for the formation of the world’s coldest, densest waters and act to prevent the flow of land-fast ice into the ocean, which would result in sea-level rise. Their disintegration is thus of great scientific interest and growing public concern. It has been hypothesized that ice shelf breakup may result from ice shelf thinning, which can be caused by densification through surface processes, a decrease in grounded ice flow, or increased surface or basal melting. To investigate whether ice shelf melting may be contributing to the collapse of LCIS, we collected full depth profiles of seawater samples at 17 stations in the vicinity of LCIS in January 2019 during the Weddell Sea Expedition. To investigate the formation processes and distribution of water masses, as well as identify regions of ice shelf melt, the samples were measured for seawater oxygen isotopic composition (δ18O) using a Picarro Cavity Ring-Down Spectroscope (CRDS). The isotope data provide little evidence of large-scale surface or basal ice shelf melting, with basal ice shelf melt constituting a maximum of 0.5% of the Ice Shelf Water (ISW) observed in the vicinity of LCIS. One implication of this is that surface and basal melting may not be the primary factor driving the collapse of LCIS, although more data and further study are required to confirm this. In addition, the isotope data are consistent with previous work suggesting that the onshore advection of warm offshore waters occurs via the Jason Trough, a remnant depression in the seafloor caused by the flow of a palaeo-ice stream. This, in combination with the observation (based on incorporating seawater δ18O into a temperature-salinity-oxygen mass balance model) that the outflow of ISW occurs primarily to the north of the study region, supports a clockwise circulation pattern in the vicinity of LCIS.

How to cite: Mirkin, J., West, A., Hutchinson, K., Flynn, R., and Fawcett, S.: Meltwater and circulation characteristics adjacent to Larsen C ice shelf: insights from seawater oxygen isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18507, https://doi.org/10.5194/egusphere-egu2020-18507, 2020.

D2479 |
Aleksandr Montelli, Christine Batchelor, Dag Ottesen, Julian Dowdeswell, Jeff Evans, and Evelyn Dowdeswell

Linear to curvilinear depressions interpreted as iceberg ploughmarks are identified on the continental shelf beyond Larsen C Ice Shelf in about 350 m water depth using multibeam echo-sounding at sub-metre horizontal resolution. Detailed imaging of ploughmark morphology demonstrates the presence of irregularly spaced ridges extending across the full ploughmark width. These ridges have an arcuate shape in plan-view, are up to 2 m high, 20-40 m wide, show occasional presence of subdued debris-flow lobes on their distal side and have an asymmetric cross-profile in which the seafloor deepens beyond their slightly steeper side. The ridges are interpreted to have been produced when the iceberg moved backwards under the falling tide, which pushed up a ridge of sediment behind the iceberg keel, before it continued on its original trajectory under the rising tide. Similar features, which we term ‘iceberg tidal ridges’, can be identified at lower resolution on bathymetric and three-dimensional seismic data from the mid-Norwegian margin, suggesting the broader implications of the interpretations presented here. For example, the mapping of delicate ridges preserved within iceberg ploughmarks can be used to reconstruct past oceanic circulation including the former direction and strength of ocean currents.

How to cite: Montelli, A., Batchelor, C., Ottesen, D., Dowdeswell, J., Evans, J., and Dowdeswell, E.: Tidally influenced iceberg motion: sub-metre resolution imaging of iceberg ploughmarks using autonomous underwater vehicles in the Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2726, https://doi.org/10.5194/egusphere-egu2020-2726, 2020.

D2480 |
Krissy Reeve, Torsten Kanzow, Mario Hoppema, Olaf Boebel, Volker Strass, Walter Geibert, and Rüdiger Gerdes

The Weddell Gyre is an important region in that it feeds source water masses (and thus heat) toward the ice-shelves, and exports locally and remotely formed dense water masses to the global abyssal ocean. Argo float profiles and trajectories were implemented to capture the large-scale, long-term mean circulation of the entire Weddell Gyre, from which the heat budget has been diagnosed for a layer within Warm Deep Water (WDW), the main heat source to the Weddell Gyre. We show that heat is horizontally advected into the southern limb of the Weddell Gyre, and then removed from the southern limb by horizontal turbulent diffusion (1) northwards towards the gyre interior, and (2) southwards towards the ice shelves. Since the gyre is cyclonic, the heat that is turbulently diffused into the gyre interior is subsequently brought closer to the surface by upwelling. Upwelling is thus an important yet poorly understood feature of the dynamics of the Weddell Gyre. This study marks the beginnings of a project focused on improved understanding of the role of upwelling within the Weddell Gyre, and investigating the role of turbulent diffusion in redistributing heat towards the central gyre interior, as well as towards the ice shelves of Antarctica.

How to cite: Reeve, K., Kanzow, T., Hoppema, M., Boebel, O., Strass, V., Geibert, W., and Gerdes, R.: The horizontal circulation, upwelling and heat budget of the Weddell Gyre: an observation perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16776, https://doi.org/10.5194/egusphere-egu2020-16776, 2020.

D2481 |
Robert Larter, Julia Wellner, Alastair Graham, Claus-Dieter Hillenbrand, Kelly Hogan, Frank Nitsche, James Smith, Lars Boehme, Mark Barham, John Anderson, Rebecca Totten, Lauren Simkins, Karen Heywood, and Erin Pettit and the NBP20-02 Shipboard Scientific Party

Thwaites Glacier (TG) is more vulnerable to unstable retreat than any other part of the West Antarctic Ice Sheet. This is due to its upstream-dipping bed, the absence of a large ice shelf buttressing its flow and the deep bathymetric troughs that route relatively warm Circumpolar Deep Water (CDW) to its margin. Over the past 30 years the mass balance of TG has become increasingly negative, suggesting that unstable retreat may have already begun. The International Thwaites Glacier Collaboration (ITGC) is an initiative jointly funded by the US National Science Foundation and the Natural Environment Research Council in the UK to improve knowledge of the boundary conditions and drivers of change at TG in order improve projections of its future contribution to sea level. The ITGC is funding a range of projects that are conducting on-ice and marine research, and applying numerical models to utilize results in order to predict how the glacier will change and contribute to sea level over coming decades to centuries.

RV Nathaniel B Palmer cruise NBP20-02, taking place from January­ to March 2020, will be the second ITGC multi-disciplinary research cruise, building on results from NBP19-02, which took place last year. Thwaites Offshore Research Project (THOR) aims during NBP20-02 include: extending the bathymetric survey in front of TG, collecting sediment cores at sites selected from the survey data, and acquiring high-resolution seismic profiles to determine the properties of the former bed of TG that is now exposed. The detailed bathymetric data will reveal the dimensions and routing of troughs that conduct CDW to the glacier front and will image seabed landforms that provide information about past ice flow and processes at the bed when TG was more extensive. The sediment cores, together with ones collected recently beneath the ice shelf via hot-water drilled holes, will be analysed to establish a history of TG retreat, subglacial meltwater release, and CDW incursions extending back over decades, centuries and millennia before the short instrumental record. Thwaites-Amundsen Regional Survey and Network Project (TARSAN) researchers will reach islands and ice floes via zodiac boats to attach satellite data relay loggers to Elephant and Weddell seals. The loggers record ocean temperature and salinity during the seals’ dives, greatly increasing the spatial extent and time span of oceanographic observations. In addition to work that is part of the THOR and TARSAN projects, another cruise objective is to recover and redeploy long-term oceanographic moorings in the Amundsen Sea. We will present initial results from NBP20-02.

How to cite: Larter, R., Wellner, J., Graham, A., Hillenbrand, C.-D., Hogan, K., Nitsche, F., Smith, J., Boehme, L., Barham, M., Anderson, J., Totten, R., Simkins, L., Heywood, K., and Pettit, E. and the NBP20-02 Shipboard Scientific Party: Initial results from International Thwaites Glacier Collaboration cruise NBP20-02, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17529, https://doi.org/10.5194/egusphere-egu2020-17529, 2020.

D2482 |
Adriano Lemos and Céline Heuzé

The sea ice thickness in the Weddell Sea during the austral winter normally exceeds 1 m, but in the case of a polynya, this thickness decreases to 10 cm or less. There are two theories as to why the Weddell Polynya opens: 1) comparatively warm oceanic water upwelling from its nominal depth of several hundred metres to the surface where it melts the sea ice from underneath; or 2) opening of a lead by a passing storm, lead which will then be maintained open either by the atmosphere or ocean and grow. The objective of this study is to estimate how long in advance the recent Weddell Polynya opening could have been detected by synthetic aperture radar (SAR) images due to the decrease of the sea ice thickness and/or early appearance of leads. We use high temporal and spatial resolution SAR images from the Sentinel-1 constellation (C-band) and ALOS2 (L-band) during the austral winters 2014-2018. We use an adapted version of the algorithm developed by Aldenhoff et al. (2018) to monitor changes in sea ice thickness over the polynya region. The algorithm detects the transition of the sea ice thickness through changes in small scale surface roughness and thus reduced backscatter, and allowing us to distinguish three different categories: ice, thin ice, and open water. The transition from ice to thin ice and then to open water indicates that the polynya is melted from under, whereas a direct transition from ice to open water will reveal leads. The high resolution and good coverage of the SAR imagery, and a combined effort of different satellites sensors (e.g. infrared and microwave sensors), opens the possibility of an early detection of Weddell Polynya opening.

How to cite: Lemos, A. and Heuzé, C.: Early detection of the Weddell polynya re-opening using SAR imagery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10790, https://doi.org/10.5194/egusphere-egu2020-10790, 2020.

D2483 |
Tom Jordan, David Porter, Kirsty Tinto, Romain Millan, Atsuhiro Muto, Kelly Hogan, Robert Larter, Alastair Graham, and John Paden

Ice shelf buttressing plays a critical role in the long-term stability of ice sheets. The underlying bathymetry and cavity thickness therefore is a key to accurate models of future ice sheet evolution. However, direct observation of sub-ice shelf bathymetry is time consuming, logistically risky, and in some areas simply not possible, meaning there is a blind-spot in our understanding of this key system. Here we use airborne gravity anomaly data to provide new estimates of sub-ice shelf bathymetry outboard of the rapidly changing West Antarctic Thwaites Glacier, and beneath the adjacent Dotson and Crosson Ice Shelves. These regions are of especial interest as the low-lying inland reverse slope of the Thwaites glacier system makes it vulnerable to collapse through marine ice sheet instability, with rapid grounding-line retreat observed since 1993 suggesting this process may be underway. Our results confirm a major marine channel > 800 m deep extends to the front of Thwaites Glacier, while the adjacent ice shelves are underlain by more complex bathymetry. Comparison of our new bathymetry with ice shelf draft reveals that ice shelves formed since 1993 comprise a distinct population where the draft conforms closely to the underlying bathymetry, unlike the older ice shelves which show a more uniform depth of the ice base. This indicates that despite rapid basal melting in some areas, these “new” ice shelves are not yet in equilibrium with the underlying ocean system. We propose qualitative models of how this transient ice-shelf configuration may have developed, but further investigation is required to constrain the longevity and full impact of these newly recognised systems.

How to cite: Jordan, T., Porter, D., Tinto, K., Millan, R., Muto, A., Hogan, K., Larter, R., Graham, A., and Paden, J.: Two ice shelf populations revealed in new gravity- derived bathymetry for the Thwaites, Crosson and Dotson ice shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19603, https://doi.org/10.5194/egusphere-egu2020-19603, 2020.

D2484 |
Erin Pettit, Atsu Muto, Christian Wild, Karen Alley, Ted Scambos, Bruce Wallin, Martin Truffer, and Dale Pomraning

As part of the International Thwaites Glacier Collaboration (ITGC) field activity in West Antarctica for the 2019-2020 season, the Thwaites-Amundsen Regional Survey and Network (TARSAN) team drilled boreholes using hot water, deployed long-term instruments, and gathered several ground-based geophysical data sets to assess the ice-shelf stability and evolution.

The Thwaites Eastern Ice Shelf is an important buttress for a broad (25 km) section of Thwaites Glacier outflow and is restrained at present by a few pinning points at the northwestern edge of the shelf. The grounding line of this buttress has retreated within the last 5 years indicating instability. Recent imagery shows major new rifting and shearing within the ice shelf.

In the Dotson-Crosson Ice Shelf (a single ice shelf with a rapidly evolving central region that has thinned and ungrounded over the past 80 years), satellite data show significant ice flow speed and direction changes, as well as retreating grounding lines where tributary glaciers start to float and where ice flows over and around isolated bedrock pinning points. A complex geometry of deep seafloor troughs underlie the central ice-shelf area which lies at the convergence of the two major troughs that extend to the continental shelf edge at two widely separated locations (roughly 103°W and 117°W longitude along the continental shelf break).

We surveyed the central Thwaites Eastern Ice Shelf (‘Cavity Camp’, 75.05°S, 105.58°W) and central Dotson-Crosson Ice Shelf (`Upper Dotson’, 74.87°S, 112.20°W) to the extent possible considering site safety and scientific interest. Cavity Camp is located approximately 17 km down-flow of the 2011 Thwaites Glacier grounding line. Ground-penetrating radar data show the ice thickness near Cavity Camp to be 300m, which is ~200m thinner than in 2007 estimated from hydrostatic assumption using altimetry analysis by other researchers. The seafloor below Cavity Camp is 816m, based on pressure from a CTD profile (a ~540 m water column and ~40m of firn).   

Across the central Dotson-Crosson Ice Shelf, a network of basal channels creates variable thinning rates from near-zero to over 30 m/yr (estimated in several previous remote-sensing-based studies). Ice thickness near our camp over a subglacial channel is 390m and the ice has been thinning at ~25 m/yr estimated from satellite data. Seafloor elevation at the Dotson site is estimated at -570 m, but seismic surveys suggest that the seabed topography varies considerably beneath Dotson. 

On each ice shelf, we conducted ~200 km of multi-frequency ground-penetrating radar profiles. We also conducted 46 (Thwaites) and 17 (Dotson) autonomous phase-tracking radio echo-sounding (ApRES) repeat point measurements, as well as 37 (Thwaites) and more than 20 (Dotson) active-seismic spot soundings to characterize the sub-ice-shelf cavity shape, thinning rates, basal ice structures, and ocean circulation. We deployed two Automated Meteorology Ice Geophysics Ocean observation Systems (AMIGOS-III stations) on the Thwaites Ice Shelf that include a suite of surface sensors, a fiber-optic-based thermal profiler, and an ocean mooring. Additionally, we deployed four long-term ApRES on the two ice shelves to monitor temporal variability in ice melt.

How to cite: Pettit, E., Muto, A., Wild, C., Alley, K., Scambos, T., Wallin, B., Truffer, M., and Pomraning, D.: Thwaites and Dotson Ice Shelves: Field Site Selection and Early Results of Field Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1331, https://doi.org/10.5194/egusphere-egu2020-1331, 2020.

D2485 |
Michelle Maclennan and Jan Lenaerts

High snowfall events on Thwaites Glacier are a key influencer of its ice mass change. In this study, we diagnose the mechanisms for orographic precipitation on Thwaites Glacier by analyzing the atmospheric conditions that lead to high snowfall events. A high-resolution regional climate model, RACMO2, is used in conjunction with MERRA-2 and ERA5 reanalysis to map snowfall and associated atmospheric conditions over the Amundsen Sea Embayment. We examine these conditions during high snowfall events over Thwaites Glacier to characterize the drivers of the precipitation and their spatial and temporal variability. Then we examine the seasonal differences in the associated weather patterns and their correlations with El Nino Southern Oscillation and the Southern Annular Mode. Understanding the large-scale atmospheric drivers of snowfall events allows us to recognize how these atmospheric drivers and consequent snowfall climatology will change in the future, which will ultimately improve predictions of accumulation on Thwaites Glacier.

How to cite: Maclennan, M. and Lenaerts, J.: Large-Scale Atmospheric Drivers of Snowfall on Thwaites Glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12441, https://doi.org/10.5194/egusphere-egu2020-12441, 2020.